﻿<h1>Pointers in Go</h1>

<p>
Although Go absorbs many features from all kinds of other languages,
Go is mainly viewed as a C family language.
One evidence is Go also supports pointers.
Go pointers and C pointers are much similar in many aspects,
but there are also some differences between Go pointers and C pointers.
This article will list all kinds of concepts and details related to pointers in Go.
</p>

<h3>Memory Addresses</h3>

<p>
A memory address means an offset (number of bytes) from the start point of
the whole memory managed by a system (generally, operating system).
</p>

<p>
Generally, a memory address is stored as an unsigned native (integer) word.
The size of a native word is 4 (bytes) on 32-bit architectures and 8 (bytes)
on 64-bit architectures.
So the theoretical maximum memory space size is 2<sup>32</sup> bytes,
a.k.a. 4GB (1GB == 2<sup>30</sup> bytes), on 32-bit architectures,
and is 2<sup>34</sup>GB (16 exabytes) on 64-bit architectures.
</p>

<p>
Memory addresses are often represented with hex integer literals,
such as <code>0x1234CDEF</code>.
</p>

<h3>Value Addresses</h3>

<p>
The address of a value means the start address of the memory segment
occupied by the <a href="value-part.html">direct part</a> of the value.
</p>

<h3>What Are Pointers?</h3>

<p>
Pointer is one kind of type in Go. A pointer is a value of some pointer type.
A pointer value can store a memory address.
In fact, we often call a memory address as a pointer, and vice versa.
</p>

<p>
Generally, the stored memory address in a pointer
is the address of another value.
Unlike C language, for safety reason,
there are some restrictions made for Go pointers.
Please read the following sections for details.
</p>

<h3>Go Pointer Types and Values</h3>

<div>
<p>
In Go, a non-defined pointer type can be represented with <code>*T</code>,
where <code>T</code> can be an arbitrary type.
Type <code>T</code> is called the base type of pointer type <code>*T</code>.
</p>

<p>
We can declare defined pointer types, but generally,
it’s not recommended to use defined pointer types,
for non-defined pointer types have better readabilities.
</p>

<p>
If the <a href="type-system-overview.html#underlying-type">underlying type</a>
of a defined pointer type is <code>*T</code>,
then the base type of the defined pointer type is <code>T</code>.
</p>

<p>
Two non-defined pointer types with the same base type are the same type.
</p>

Example:
<pre class="line-numbers"><code class="language-go">*int  // A non-defined pointer type whose base type is int.
**int // A non-defined pointer type whose base type is *int.

// Ptr is a defined pointer type whose base type is int.
type Ptr *int
// PP is a defined pointer type whose base type is Ptr.
type PP *Ptr
</code></pre>

<p>
</p>

<p>
Zero values of any pointer types are represented with the predeclared <code>nil</code>.
No addresses are stored in nil pointer values.
</p>

<p>
A value of a pointer type whose base type is <code>T</code> can only store the addresses of values of type <code>T</code>.
</p>

</div>

<h3>About the Word "Reference"</h3>

<p>
In Go 101, the word "reference" indicates a relation.
For example, if a pointer value stores the address of another value,
then we can say the pointer value (directly) references the other value,
and the other value has at least one reference.
The uses of the word "reference" in Go 101 are consistent with Go specification.
</p>

<p>
When a pointer value references another value, we also often say the pointer value points to the other value.
</p>

<h3>How to Get a Pointer Value and What Are Addressable Values?</h3>

<div>
There are two ways to get a non-nil pointer value.
<ol>
<li>
	The built-in <code>new</code> function can be used to allocate memory for a value
	of any type. <code>new(T)</code> will allocate memory for a <code>T</code> value
	(an anonymous variable) and return the address of the <code>T</code> value.
	The allocated value is a zero value of type <code>T</code>.
	The returned address is viewed as a pointer value of type <code>*T</code>.
</li>
<li>
	We can also take the addresses of values which are addressable in Go.
	For an addressable value <code>t</code> of type <code>T</code>,
	we can use the expression <code>&amp;t</code> to take the address of <code>t</code>,
	where <code>&amp;</code> is the operator to take value addresses.
	The type of <code>&amp;t</code> is viewed as <code>*T</code>.
</li>
</ol>
</div>

<p>
Generally speaking, an addressable value means a value which is hosted at somewhere in memory.
Currently, we just need to know that all variables are addressable,
whereas constants, function calls and explicit conversion results are all unaddressable.
When a variable is declared, Go runtime will allocate a piece of memory for the variable.
The starting address of that piece of memory is the address of the variable.
</p>

<p>
We will learn other addressable and unaddressable values from other articles later.
If you have already been familiar with Go, you can read
<a href="summaries.html#not-addressable">this summary</a> to get
the lists of addressable and unaddressable values in Go.
</p>

<p>
The next section will show an example on how to get pointer values.
</p>

<h3>Pointer Dereference</h3>

<p>
Given a pointer value <code>p</code> of a pointer type whose base type is <code>T</code>,
how can you get the value at the address stored in the pointer (a.k.a., the value
being referenced by the pointer)? Just use the expression <code>*p</code>,
where <code>*</code> is called dereference operator.
<code>*p</code> is called the dereference of pointer <code>p</code>.
Pointer dereference is the inverse process of address taking.
The result of <code>*p</code> is a value of type <code>T</code> (the base type of the type of <code>p</code>).
</p>

<p>
Dereferencing a nil pointer causes a runtime panic.
</p>

<div>
The following program shows some address taking
and pointer dereference examples:
<pre class="line-numbers"><code class="language-go">package main

import "fmt"

func main() {
	p0 := new(int)   // p0 points to a zero int value.
	fmt.Println(p0)  // (a hex address string)
	fmt.Println(*p0) // 0

	// x is a copy of the value at
	// the address stored in p0.
	x := *p0
	// Both take the address of x.
	// x, *p1 and *p2 represent the same value.
	p1, p2 := &x, &x
	fmt.Println(p1 == p2) // true
	fmt.Println(p0 == p1) // false
	p3 := &*p0 // <=> p3 := &(*p0) <=> p3 := p0
	// Now, p3 and p0 store the same address.
	fmt.Println(p0 == p3) // true
	*p0, *p1 = 123, 789
	fmt.Println(*p2, x, *p3) // 789 789 123

	fmt.Printf("%T, %T \n", *p0, x) // int, int
	fmt.Printf("%T, %T \n", p0, p1) // *int, *int
}
</code></pre>

<p>
</p>

<p>
The following picture depicts the relations of the values
used in the above program.

<div class="text-center">
<img src="res/pointer.png" alt="pointer value"></img>
</div>
</p>
</div>

<h3>Why Do We Need Pointers?</h3>

<div>
Let's view an example firstly.

<pre class="line-numbers"><code class="language-go">package main

import "fmt"

func double(x int) {
	x += x
}

func main() {
	var a = 3
	double(a)
	fmt.Println(a) // 3
}
</code></pre>

<p>
The <code>double</code> function in the above example is expected to modify
the input argument by doubling it. However, it fails. Why?
Because all value assignments, including function argument passing, are value copying in Go.
What the <code>double</code> function modified is a copy (<code>x</code>) of variable <code>a</code> but not variable <code>a</code>.
</p>

One solution to fix the above <code>double</code> function is let it return
the modification result. This solution doesn't always work for all scenarios.
The following example shows another solution, by using a pointer parameter.

<pre class="line-numbers"><code class="language-go">package main

import "fmt"

func double(x *int) {
	*x += *x
	x = nil // the line is just for explanation purpose
}

func main() {
	var a = 3
	double(&a)
	fmt.Println(a) // 6
	p := &a
	double(p)
	fmt.Println(a, p == nil) // 12 false
}
</code></pre>

<p>
We can find that, by changing the parameter to a pointer type,
the passed pointer argument <code>&amp;a</code> and its copy <code>x</code> used in the function body both reference the same value,
so the modification on <code>*x</code> is equivalent to a modification on <code>*p</code>, a.k.a., variable <code>a</code>.
In other words, the modification in the <code>double</code> function body can be reflected out of the function now.
</p>

<p>
Surely, the modification of the copy of the passed pointer argument itself
still can't be reflected on the passed pointer argument.
After the second <code>double</code> function call, the local pointer
<code>p</code> doesn't get modified to <code>nil</code>.
</p>

<p>
In short, pointers provide indirect ways to access some values.
Many languages do not have the concept of pointers.
However, pointers are just hidden under other concepts in those languages.
</p>

</div>

<h3>Return Pointers of Local Variables Is Safe in Go</h3>

<div>
Unlike C language, Go is a language supporting garbage collection,
so return the address of a local variable is absolutely safe in Go.
<pre class="line-numbers"><code class="language-go">func newInt() *int {
	a := 3
	return &a
}
</code></pre>
</div>

<a class="anchor" id="restrictions"></a>
<h3>Restrictions on Pointers in Go</h3>

<p>
For safety reasons, Go makes some restrictions to pointers (comparing to pointers in C language).
By applying these restrictions, Go keeps the benefits of pointers,
and avoids the dangerousness of pointers at the same time.
</p>

<h4>Go pointer values don't support arithmetic operations</h4>

<div>
<p>
In Go, pointers can't do arithmetic operations.
For a pointer <code>p</code>, <code>p++</code> and <code>p-2</code> are both illegal.
</p>

<p>
If <code>p</code> is a pointer to a numeric value, compilers will view
<code>*p++</code> is a legal statement and treat it as <code>(*p)++</code>.
In other words, the precedence of the address-taken operator <code>&amp;</code>
and the pointer dereference operator <code>*</code> is higher than
the increment operator <code>++</code> and decrement operator <code>--</code>.
</p>

Example:
<pre class="line-numbers"><code class="language-go">package main

import "fmt"

func main() {
	a := int64(5)
	p := &a

	// The following two lines don't compile.
	/*
	p++
	p = (&a) + 8
	*/

	*p++
	fmt.Println(*p, a)   // 6 6
	fmt.Println(p == &a) // true

	*&a++
	*&*&a++
	**&p++
	*&*p++
	fmt.Println(*p, a) // 10 10
}
</code></pre>
</div>

<h4>A pointer value can't be converted to an arbitrary pointer type</h4>

<div>
In Go, a pointer value of pointer type <code>T1</code> can be directly and explicitly converted
to another pointer type <code>T2</code> only if either of the following two conditions
is get satisfied.
<ol>
<li>
	The underlying types of type <code>T1</code> and <code>T2</code> are identical (ignoring struct tags),
	in particular if either <code>T1</code> and <code>T2</code> is a
	<a href="type-system-overview.html#non-defined-type">non-defined</a> type
	and their underlying types are identical (considering struct tags),
	then the conversion can be implicit.
	Struct types and values will be explained in
	<a href="struct.html">the next article</a>.
</li>
<li>
	Type <code>T1</code> and <code>T2</code> are both non-defined pointer types and
	the underlying types of their base types are identical (ignoring struct tags).
</li>
</ol>

For example, for the below shown pointer types:
<pre class="line-numbers"><code class="language-go">type MyInt int64
type Ta    *int64
type Tb    *MyInt
</code></pre>

the following facts exist:
<ol>
<li>
	values of type <code>*int64</code> can be implicitly converted to type <code>Ta</code>,
	and vice versa, for their underlying types are both <code>*int64</code>.
</li>
<li>
	values of type <code>*MyInt</code> can be implicitly converted to type <code>Tb</code>,
	and vice versa, for their underlying types are both <code>*MyInt</code>.
</li>
<li>
	values of type <code>*MyInt</code> can be explicitly converted to type <code>*int64</code>,
	and vice versa, for they are both non-defined and the underlying types of their base types are both <code>int64</code>.
</li>
<li>
	values of type <code>Ta</code> can't be directly converted to type <code>Tb</code>,
	even if explicitly.
	However, by the just listed first three facts, a value <code>pa</code> of
	type <code>Ta</code> can be indirectly converted to type <code>Tb</code>
	by nesting three explicit conversions, <code>Tb((*MyInt)((*int64)(pa)))</code>.
</li>
</ol>

<p>
None values of these pointer types can be converted to type <code>*uint64</code>, in any safe ways.
</p>
</div>

<h4>A pointer value can't be compared with values of an arbitrary pointer type</h4>

<div>
In Go, pointers can be compared with <code>==</code> and <code>!=</code> operators.
Two Go pointer values can only be compared if either of the following three conditions
are satisfied.
<ol>
<li>
	The types of the two Go pointers are identical.
</li>
<li>
	One pointer value can be implicitly converted to the pointer type of the other.
	In other words, the underlying types of the two types must be identical
	and either of the two types of the two Go pointers must be an undefined type.
</li>
<li>
	One and only one of the two pointers is represented with the bare (untyped) <code>nil</code> identifier.
</li>
</ol>

Example:
<pre class="line-numbers"><code class="language-go">package main

func main() {
	type MyInt int64
	type Ta    *int64
	type Tb    *MyInt

	// 4 nil pointers of different types.
	var pa0 Ta
	var pa1 *int64
	var pb0 Tb
	var pb1 *MyInt

	// The following 6 lines all compile okay.
	// The comparison results are all true.
	_ = pa0 == pa1
	_ = pb0 == pb1
	_ = pa0 == nil
	_ = pa1 == nil
	_ = pb0 == nil
	_ = pb1 == nil

	// None of the following 3 lines compile ok.
	/*
	_ = pa0 == pb0
	_ = pa1 == pb1
	_ = pa0 == Tb(nil)
	*/
}
</code></pre>
<p>
</p>
</div>

<h4>A pointer value can't be assigned to pointer values of other pointer types</h4>

<p>
The conditions to assign a pointer value to another pointer value are the
same as the conditions to compare a pointer value to another pointer value,
which are listed above.
</p>

<h3>It's Possible to Break the Go Pointer Restrictions</h3>

<p>
As the start of this article has mentioned,
the mechanisms (specifically, the <code>unsafe.Pointer</code> type)
provided by <a href="unsafe.html">the <code>unsafe</code> standard package</a>
can be used to break the restrictions made for pointers in Go.
The <code>unsafe.Pointer</code> type is like the <code>void*</code> in C.
In general the unsafe ways are not recommended to use.
</p>




